1. Field of the Invention:
The present invention relates to a controller for sensing the presence and absence of a fluid and particularly relates to a controller for sensing the level of a containerized fluid.
Level controllers for sensing the level of a fluid are well known in the prior art. The prior art utilizes metallic sheathed electric heating elements with a bi-metallic thermal switch either embedded within or attached to the electric heating element. The bi-metal switch is heated by the electric heating element. When the unit including the switch and heating element is immersed in a fluid, the fluid cools the bi-metallic switch sufficiently to prevent actuation thereof. When the level of the fluid drops to a predetermined level, at which the bi-metallic switch is no longer immersed in the fluid, the fluid no longer cools the bi-metallic heating element and the element is then heated by the heater thereby actuating the bi-metallic switch. Bi-metallic thermal switches, due to their inherent design, have a limited temperature switching range, and due to their relative mass, are restricted to predetermined positions. Preferably bi-metallic switches are mounted horizontally along their axis. The metallic construction of the bi-metallic switch also limits its application in corrosive fluids. Additionally, failure of a bi-metallic switch is not fail safe due to the fact that the switch can fail in its open or closed condition or the heater can fail without actuation of the bi-metallic switch. Such a failure can go undetected and can have catastrophic consequences when utilized to sense the level of a fluid in a container.
It is also known in the prior art to utilize a heater element and two matched resistance temperature detectors which are disposed in a bridge circuit. If the temperature sensors are unevenly heated, such uneven heating is sensed by the bridge circuit. The resistance temperature detectors can be utilized for flow detection or liquid level/interface detection.